In plastic films, it is possible to continuously produce a film having a great area which cannot be achieved by other materials. Utilizing the characteristics such as strength, durability, transparency, flexibility and separation property between surface side and back surface side, plastic films are used in fields such as agricultural, package and architectural fields which require large amounts of plastic film. Specifically, because biaxially oriented polyester films have excellent mechanical, thermal, electrical and chemical properties, they are used for various fields, and particularly, the usefulness thereof as base films of magnetic tapes is unrivaled by films made from other materials. Recently, such base films have been required to be thinner in order to lighten and miniaturize equipment, enable longer recordings, and therefore, the base films have been required to be further strengthened.
Further, in the fields of heat transfer ribbons, capacitors and thermo-stencil printing plates, thin films have been strongly required, and similarly the films used have been required to be further strengthened.
As a process for strengthening a biaxially oriented polyester film, generally known is a so-called longitudinal re-stretching process for re-stretching a biaxially stretched film in the longitudinal direction and providing a high lengthwise strength to the film (for example, JP-B-SHO 42-9270, JP-B-SHO 43-3040, JP-A-SHO 46-1119 and JP-A-SHO 46-1120). Further, in a case where a film is required to be further strengthened in the transverse direction as well as in the longitudinal direction, there is a so-called longitudinal and transverse re-stretching process for re-stretching the film in the transverse direction after the longitudinal re-stretching (for example, JP-A-SHO 50-133276 and JP-A-SHO 55-22915).
However, even in such a longitudinal re-stretching process, although the principal orientation axis is present in the longitudinal direction, the orientation of the whole of the film is not so great, and irregularity in thickness and irregularities in properties in the transverse direction are not greatly improved.
Moreover, in a process having the above-described longitudinal re-stretching process, because generally a thin film having a large width is stretched by rollers, if an edge portion of the film is thin, the neck down at the time of stretching is violent, thereby causing serious problems such as deterioration of irregularity in thickness, irregularity in other properties, and generation of scratches. In order to avoid these problems, generally edges of a cast film are formed thick and a holding force is provided thereto for preventing the neck down.
In a case where longitudinal re-stretching is performed, a cast film must be formed so that a ratio (A/B) of the maximum thickness (A) of the edge portion to the thickness (B) of the central portion in the transverse direction of the cast film is a value around 10, although the ratio to be controlled depends upon the thickness of the central portion.
However, if the difference between the thickness of the edge portion and the thickness of the central portion is great as described above, although a neck down can be lessened problems occur in that the edge portion having a large thickness is insufficiently pre-heated, thereby causing frequent film breakage, and irregularity in properties in the transverse direction due to the temperature difference becomes great. Further, because the edge portions are finally trimmed away from a film product portion, thick edge portions are not desired also from the viewpoint of yield.